eoje gongwoneseo georeossneunde jeongmal joyonghadeora.

-는데 connects two clauses and usually sets up background / context for what comes next, often with a mild sense of “and then / and (I noticed that) …” or sometimes contrast.

• Here, 어제 공원에서 걸었는데 = “Yesterday, (I) walked in the park, and…” (setting the scene)
• Then the main point follows: 정말 조용하더라 = “it was really quiet (I noticed).” It’s not as “because”/causal as English because, but it can feel like “when I did X, I found Y.”

에서 marks the place where an action happens.

• 공원에서 걸었어요 = “I walked in/at the park.” 에 often marks a destination or a location of existence.
• 공원에 갔어요 = “I went to the park.”
• 공원에 있어요 = “I’m at the park.” So with 걷다 (to walk), 에서 is the natural choice.

걷다 is a ㄷ-irregular verb: when a vowel follows, ㄷ changes to ㄹ.

• 걷 + 어요 → 걸어요
• 걷 + 었어요 → 걸었어요 So 걸었는데 is 걷다 in past tense + -는데.

-더라 expresses that the speaker is recounting something they personally experienced/observed (a remembered realization).

• 조용하더라 feels like: “(I noticed/realized) it was quiet.” Compared to:
• 조용했어: plain past fact (“It was quiet.”)
• 조용하더라: past observation with a “remembering/realizing” tone (“It turned out to be quiet / I found it quiet.”)

Normally -더라 is for something you directly saw/experienced. If you only heard it, Korean usually uses hearsay forms like:

• 조용하대(요) = “They say it’s quiet.”
• 조용했다더라 = “I heard it was quiet.” So 조용하더라 strongly implies you personally perceived the quietness.

Korean often omits subjects when they’re obvious from context. This sentence most naturally implies “I”:

• (나는) 어제 공원에서 걸었는데… You can add a subject if needed:
• 나는 어제 공원에서 걸었는데 정말 조용하더라. Or change it to someone else:
• 친구가 어제 공원에서 걸었는데 정말 조용하더라. (Now it’s clearly about the friend.)

Yes, 정말 means really / truly and emphasizes the following description. Common placements:

• 정말 조용하더라 (most natural here)
• 어제 공원에서 정말 걸었는데… (sounds odd—emphasis on really walked) So 정말 is best placed right before what you want to intensify: 조용하다.

This is casual / plain style (often used in storytelling with friends). Polite versions:

• 어제 공원에서 걸었는데 정말 조용하더라고요.
• 어제 공원에서 걸었는데 정말 조용하더라구요. (common casual spelling) You can also go more neutral:
• 어제 공원에서 걸었는데 정말 조용했어요. (polite but loses the “I noticed” feel of -더라)

Both can come from observation, but the nuance differs:

• 조용하더라: simple recollection/realization (“I found it was quiet.”)
• 조용하던데: often sets up contrast or implication (“It was quiet though / It was quiet when I saw it, so…”), and it frequently invites a follow-up like disagreement, surprise, or comparison. Example:
• 공원은 조용하던데, 여긴 왜 이렇게 시끄러워? = “The park was quiet, so why is it so noisy here?”

In careful spelling it’s 걸었는데, but in pronunciation the ㄷ (from 었) before ㄴ undergoes nasal assimilation:

• 걸었는데 → often pronounced like [거런데] This is similar to:
• 있었는데 → [이썬는데] / [이선는데] (heard as a doubled ㄴ sound)

Yes, both work, but they’re slightly different:

• 걸었는데 = “(I) walked” (focus on walking itself)
• 산책했는데 = “(I) took a walk / went for a stroll” (more like a leisure activity) So 산책했는데 정말 조용하더라 can feel more like a relaxing outing than just walking.

Yes, -는데 can end a sentence in conversation, implying something like “you know…” or inviting a response:

• 어제 공원에서 걸었는데… (implies you’ll add more, or you’re prompting the listener) But in your sentence it doesn’t end—it's doing its normal job connecting to 정말 조용하더라.